1. Field of the Invention
The present invention relates to a ring gear or an internal gear that meshes with pinions of a planetary gear incorporated in an automatic transmission and also to a method of forming the internal gear.
2. Description of the Related Art
Internal gear 50 of the planetary gear incorporated in a transmission generally has a structure shown in FIG. 4. It is a gear component shaped like a bottomed cylinder whose cylindrical portion 51 comprises a gear body, the inner circumferential surface of which is formed with a plurality of teeth 51a in mesh with a pinion 54 rotatably supported on a pinion shaft 56. A bottom portion (flange portion) 52 of the internal gear 50 is shaped like a disk with uniform density and thickness and with an opening formed at the center on the inner diameter side to pass a shaft 55. The inner diameter part of the bottom portion 52 is integrally and coaxially provided with a boss portion 53. The internal gear 50 of such a construction is incorporated in the transmission by mounting the boss portion 53 on the outer circumferential surface of the shaft 55.
When the transmission operates driving the planetary gear, the gear body 51 of the internal gear 50 is subjected to vibrations resulting from errors of meshing with the pinion 54 during transmission of force. Because the boss portion 53 is fixed to the shaft 55, the internal gear 50 vibrates at the bottom portion (flange portion) 52 during rotation. With the conventional internal gear 50, however, because the bottom portion 52 has uniform density in the radial direction, the vibrations entering the gear body 51 are not so much attenuated even when they are transmitted to the bottom portion 52.
Where the internal gear is an integrally formed component by sintering, a multistage forming method that uses a plurality of divided molds is used to press and form it into the above-mentioned shape for the purpose of improving yield.
With the internal gear of the above conventional shape, however, strict control must be performed in the positions of the molds to ensure that the inner side surface of the flange portion and the inner end face of the boss portion are flush, i.e., a lower mold surface for forming the flange portion and a lower mold surface for forming the boss portion are flat and flush. It is also necessary to precisely control the density of powder to be compressed at various portions. The conventional shape of the internal gear, as described above, has a drawback of making the compression molding control complex.
Because the position control and the density control need to be performed simultaneously and strictly, any degradation of control precision, if it occurs with the conventional shape of the internal gear, may cause a sharp change in density at a joint between the split molds, i.e., the flange portion and the boss portion. This sharp density change will result in cracks in the joint, leading to a deteriorated resistance against torsional torque.
Further, the self-adjustment for smooth engagement between the internal gear and the pinion during the initial stage of operation is known to depend on the surface hardness of the internal gear as well as the pinion. To improve the initial self-adjustment for smooth engagement it has been proposed to set the surface hardness of the internal gear lower than that of the pinion.
When the error of parallelism between the rotating axes of the pinion and the internal gear exceeds an allowable range, which is caused by a clearance between the inner circumferential surface of the pinion and the outer circumferential surface of the pinion shaft and the resulting inclination between them, only a part of one of paired tooth surfaces of the pinion, which is formed as a helical gear, engages a corresponding tooth of the internal gear at a contact point. Hence, there is a possibility of a pitting occurring near both ends in the tooth trace direction of the pinion or of a flaking occurring on the outer circumferential surface of the pinion shaft. A possible countermeasure for this problem may involve enhancing the precision with which the pinion is assembled to the internal gear to reduce variations in parallelism. This method, however, increases the machining cost and there is a certain limit on the extent to which the parallelism variations can be reduced.